The present invention relates to vanes for a steam turbine. More specifically, the present invention relates to a high performance vane for use in the latter stages of a steam turbine and having an airfoil portion with improved curvature.
The steam flow path of a steam turbine is formed by a stationary cylinder and a rotor. A large number of stationary vanes are attached to the cylinder in a circumferential array and extend inward into the steam flow path. Similarly, a large number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades forms a stage. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. The blade airfoils extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached to it.
The amount of energy extracted by each stage depends on the size and shape of the vane and blade airfoils, as well as the quantity of vanes and blades in the stage. Thus, the shapes of the airfoils are an extremely important factor in the thermodynamic performance of the turbine and determining the geometry of the airfoils is a vital portion of the turbine design.
As the steam flows through the turbine its pressure drops through each succeeding stage until the desired discharge pressure is achieved. Thus, the steam properties--that is, temperature, pressure, velocity and moisture content--vary from stage to stage as the steam expands through the flow path. Consequently, each stage employs vanes and blades having an airfoil shape that is optimized for the steam conditions associated with that stage. However, within a given row the vane airfoils are identical.
Generally, the major thermodynamic losses in the vane row occur due to friction losses as the steam flows over the airfoil surface and separation of the boundary layer on the suction surface of the vane. Friction losses are minimized by shaping the airfoil so as to maintain the steam local velocity on the airfoil surface at relatively low values. Separation of the boundary layer is prevented by causing the steam to constantly accelerates as it flows toward the trailing edge of the airfoil. This constant acceleration requires that the passage between adjacent airfoils constantly converges from the vane inlet to the gauging point.
The difficulty associated with designing a steam turbine vane is exacerbated by the fact that the airfoil shape determines, in large part, the mechanical strength of the vane and its resonant frequencies, as well as the thermodynamic performance of the vane. These considerations impose constraints on the choice of vane airfoil shape. Thus, of necessity, the optimum vane airfoil shape for a given row is a matter of compromise between its mechanical and aerodynamic properties. One important constraint involves the thickness of the trailing edge portion of the airfoil. If the trailing edge is too thin, distortion can result in the airfoil as a result of the forging process by which the vanes are manufacture. However, increasing the thickness of the trailing edge can compromise the convergence necessary to prevent steam flow separation.
It is therefore desirable to provide a row of steam turbine vanes having an airfoil shape that provides a sufficiently thick trailing edge region to prevent distortion during forging but which maintains the steam velocity at relatively low values and ensures that the steam does not decelerate as it flows toward the trailing edge.